The process of producing a liquefied natural gas includes a step of preprocessing a natural gas, such as removal of acid gas and water, a subsequent step of preliminarily cooling the natural gas to, for example, approximately −40° C. using a precooling refrigerant, and a subsequent step of removing a heavy gas from the natural gas and liquefying the natural gas by cooling the gas to a temperature within the range of, for example, −155° C. to −158° C. using a primary refrigerant. Examples usable as a precooling refrigerant include a refrigerant containing propane as a main component, and examples usable as a primary refrigerant include a mixed refrigerant containing a mixture of methane, ethane, propane, and nitrogen.
These refrigerants are circulated in a vapor compression refrigeration cycle. In the refrigeration cycle, the refrigerants in gaseous form are compressed by a compressor and then cooled and liquefied by a condenser. The liquefied high-pressure refrigerants have their pressures and temperatures reduced by, for example, expansion valves or expansion turbines. The low-temperature refrigerants are gasified again into a gas by exchanging heat with the natural gas. The precooling refrigerant is also used to cool a primary refrigerant compressed by the compressor. After cooled by the precooling refrigerant, the primary refrigerant exchanges heat with the natural gas.
Patent Literature 1 describes such a gas liquefaction plant in which the following devices are disposed on one side of a pipe rack forming a pipe assembly: a precooling heat exchanging system using a first refrigerant (precooling refrigerant), a first refrigerant compressor that compresses the first refrigerant, a very-low-temperature heat exchanging system using a second refrigerant (primary refrigerant), and a second refrigerant compressor that compresses the second refrigerant.
Industrial gas turbines as large as, for example, 80 MW have been used as compressor driving sources. On the other hand, high-performance efficient smaller gas turbines of, for example, 25 MW to 60 MW, originally developed for airplanes, have recently been developed. The inventors assume that the use of multiple smaller gas turbines would enhance the design freedom.
The use of multiple smaller gas turbines in the layout disclosed in Patent Literature 1, however, makes the piping arrangement complex, requiring a wide space and increasing the size of the gas liquefaction plant.
Patent Literature 2 describes the technology involving multiple compressors driven by respective gas turbines in the refrigeration cycle for liquefying a natural gas, but does not disclose the technology for solving the problem confronted by the inventors.